Mode control for theta mode magnetrons



Dec. 24, 1968 E. J. cooK MODE CONTROL FOR G'MODE MAGNETRONS 2 Sheets-Sheet 1 Filed Feb. 7, 1966 F IG.|

INVENTOR. EDWARD J. COOK ATTORNEY Dec. 24, 1968 J, COOK 3,418,522

MODE CONTROL; FOR G-MODE MAGNETRONS Filed Feb 7, 1966 ZSheets-Sheet 2 FIG.5

I NVEN TOR.

EDWARD J. 000K ATTORNEY United States Patent 3,418,522 MODE CONTROL FOR 0 MODE MAGNETRONS Edward J. Cook, South Hamilton, Mass, assignor to Varian Associates, Palo Alto, Calif., a corporation of California Filed Feb. 7, 1966, Ser. No. 525,454 8 Claims. (Cl. 31539.63)

ABSTRACT OF THE DISCLOSURE A voltage tunable magnetron is disclosed. The magnetron includes a non-electron emissive cathode sole electrode surrounded by a reentrant periodic anode slow wave circuit to define an annular magnetron interaction region therebetween. A thermionic electron emitter is axially spaced from the sole electrode for axially injecting a stream of electrons into the interaction region for cumulative interaction with wave energy on the periodic circuit to produce an output signal. The periodic circuit is coupled to a surrounding cavity resonator and a coupling loop is provided in the resonator for coupling the composite interaction circuit to a load. The coupling to the load is sufficiently high such that the interaction circuit is heavily loaded to provide a loaded interaction circuit Q less than 50. The composite interaction circuit, as coupled to the load, is capable of supporting an interfering mode of oscillation at the signal frequency. A wave energy perturbing means, such as a conductive element or lossy element, is provided on the loaded interaction circuit at a position of relatively low amplitude of the desired mode and relatively high amplitude of the undesired mode for perturbing the undesired mode and removing it from substantial interference with the desired mode. Such perturbing means is located at only a single location as taken around the circumference of the anode circuit to prevent dividing the circuit into two or more resonant sections which could interfere with transfer of energy around the circuit.

Heretofore, 6 mode magnetron oscillators have been built which operated on the (N/Z-l) or first 6 mode. However, it was found that the power output spectrum of such a tube was discontinuous over the operating band because of the presence of a relatively high Q mode of resonance typically located near the center of the operating band. This interferring mode was the orthogonal orientation of the TM mode in the coaxially disposed cavity resonator which surrounded the periodic anode circuit of the magnetron. In contrast to the heavily coupled preferred orientation of the T M mode, the orthogonal pattern is very lightly coupled and therefore exhibits a high loaded Q.

In the present invention the undesired orthogonal mode of the coaxially disposed cavity is heavily perturbed and shifted out of the operating band of the tube by strongly perturbing the periodic circuit, as by a short circuit thereof, at a point on the circuit which corresponds to a null of the desired mode and a maximum of the undesired mode. In a preferred embodiment of the present invention, the magnetron interaction circuit comprises an interdigital line crown supported to a surrounding cavity and the perturbing means comprises a short circuit of the interdigital line at a point on its circumference which is approximately in quadrature with the output coupling means coupled to the cavity.

The principal object of the present invention is the provision of an improved 0 mode magnetron having a.

more uniform power output spectrum.

One feature of the present invention is the provision of a wave perturbing means located on the periodic circuit of the 0 mode magnetron at a point which corresponds to a null of the desired mode and a maximum of the undesired mode whereby the undesired mode is perturbed in frequency out of the operating band of the tube or otherwise suppressed or controlled.

Another feature of the present invention is the same as the preceding feature wherein the operating 0 mode of the tube is the (N/ 2-1) mode and the perturbing means is located in approximate spaced quadrature from the out put coupling means of the tube.

Another feature of the present invention is the same as any one or more of the preceding features wherein the perturbing means comprises a short circuit on the periodic magnetron interaction circuit.

Another feature of the present invention is the same as any one, or more, of the preceding features wherein the periodic circuit is a crown-supported interdigital line and the perturbing means comprises a conductive connection forming a short between adjacent fingers of the interdigital line.

Other features and advantages of the present invention become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

FIG. 1 is a schematic line diagram of a magnetron incorporating the features of the present invention;

FIG. 2 is an w-fl diagram showing the dispersion characteristics for the magnetron of FIG. 1;

FIG. 3 is a longitudinal sectional view of a magnetron incorporating the features of the present invention;

FIG. 4 is a transverse schematic line diagram of a magnetron of the type shown in FIG. 3 depicting the resonant mode patterns for the operating first 0 mode;

FIG. 5 is a plot of power output versus frequency showing the power spectrum of the prior art;

FIG. 6 is an enlarged fragmentary view of a portion of the structure of FIG. 3 delineated by line 6-6, and

FIG. 7 is an w-fi diagram showing the operating portion of the dispersion curve for the magnetron tube of FIG. 3.

Referring now to FIG. 1, there is shown a typical magnetron having a backward wave fundamental mode of operation. More specifically, the magnetron comprises an anode 1 coaxially surrounding a cathode electrode 2 and defining an annular magnetron interaction region 3 in the space between the anode 1 and the cathode 2. A periodic magnetron interaction circuit 4 is formed in the inner face of the anode 1 facing the magnetron interaction region 3 for cumulative electronic interaction with a stream of electrons provided in the interaction region 3 in the presence of an axially directed magnetic field B. The cumulative interaction between wave energy on the circuit 4 and the electron stream produces an output signal on the circuit 4 which is coupled therefrom by a suitable coupling device 5 for transmission to a suitable load, not shown.

The dispersion characteristic for the typical backward wave fundamental periodic circuit is shown in FIG. 2. More specifically, if the anode circuit 4 were not reentrant, the dispersion characteristics for the circuit would appear as the solid line 6 having a low frequency cutoff m at the 11' phase shift per section and having an upper cutoff frequency (.1 for zero phase shift per section. When the anode circuit 4 is made re-entrant, such that a wave may travel in a continuous path around the anode, the dispersion characteristic for the circuit becomes discontinuous and operation may only be had for frequencies corresponding to an integral number of whole electrical wavelengths taken around the anode circuit 4. These discontinuous points of operation are indicated by the solid vertical lines on the dispersion curve. If there are N elements in the periodic circuit 4, there will be found to be N/2 possible modes of operation for the circuit, such modes being equally spaced in p, as indicated in FIG. 2. Thus, assuming that there are eight periodic elements in the circuit, as shown in FIG. 1, there will be N/ 2 or four possible modes of operation indicated as N/Z, (N/21), (N/22, and (N/23). From the relatively steep dis persion curve 6 in the high phase shift per section range of the dispersion curve, i.e., between 11'/ 2 and 11', as shown in FIG. 2, it can be seen that a substantial amount of frequency separation is obtained between adjacent modes. 'In particular, it is seen that if operation is had on the (N/2-1) mode that the operating bandwidth from m to up; will be substantial without any interference from the two adjacent modes. These possible modes of operation other than the conventional 1r mode have become known in the art as modes. The first 6 mode, i.e., (N/2-l) is particularly attractive as its interaction impedance may be substantial.

It turns out that if the magnetron interaction circuit 4 has a single reflection thereon, it will produce additional possible 1/2 modes of operation between the integral 0 modes, namely, operation at (N/21/2, (N/23/2), (N/25/2), and (N/2-7/2). However, these half 0 modes do not present nearly as high an average interaction impedance to the electron stream as the integral modes and thus possible interference or interaction with these modes can be expected to be of a lesser magnitude than operation on one of the integral modes.

Referring now to FIG. 3, there is shown in longitudinal section a magnetron incorporating the features of the present invention. The magnetron includes a cold cathode electrode 11 axially disposed of the tube and surrounded by a magnetron interaction circuit 12. A filamentary emitter 13 is axially disposed at one end of the tube opposed to the cold cathode electrode 11 for emitting a stream of electrons. An injector electrode 14 coaxially surrounds the filamentary emitter 13 and serves to inject a stream of electrons axially of the tube into the magnetron interaction region 15 defined by the annular region between the cold cathode electrode 11 and the anode circuit 12. The magnetron tube is evacuated and the vacuum envelope is formed by metallized vacuum seals made between adjacent elements of the tube and; in particular, between the cold cathode 11, ceramic ring 16, anode 12, ceramic ring 17, and end closing ceramic plate 18.

The anode circuit 12 includes an interdigital line portion 21 closely surrounding the cold cathode electrode 11. A folded section of radial cavity 22 surrounds the interdigital line 21 and serves to support the interdigital line in what may be referred to as a crown-support manner: that is, the interdigitated fingers of the interdigital line 21 are supported at their axially spaced ends from opposite axially spaced side walls of the cavity 22. This is more clearly shown by the fragmentary view of FIG. 6.

An output coupling means 23 is coupled to the fields of the resonator 22 for extracting wave energy from the circuit 12 for propagation to a suitable load, not shown, via the intermediary of a coaxial line 24. The coupling means 23 includes an extension of the center conductor 25 of the coaxial line which extends across the cavity 22 from one side wall to the other making contact to the opposed wall and passing through an aperture in the outer wall of the cavity. A dielectric wavepermeable window member 26 coaxially surrounds the coupling center conductor 25 forming a gas-tight seal thereto.

Cavity 22 is dimensioned for a dominant mode of resonance corresponding to the 1r mode of the circuit 12. However, for the desired (N/2-1) or first 0 mode, the cavity 22 will be resonant in its first higher order mode corresponding to the TM mode, as depicted in FIG. 4. In the absence of the output coupling means 23, the TM mode may assume any position about the circumference of the cavity 22. However, when the coupling device 23 is inserted into the cavity 22 and substantial coupling is obtained to the cavity, the coupling means 23 serves to lock the mode of resonance in one position and to split it into two orthogonal components. The TM mode is characterized as having a pair of nodes or nulls taken about the circumference of the cavity, the nodes being indicated in FIG. 4 at 27. One orthogonal component of the TM mode, which is heavily coupled to the coupling means 23, will assume a position which has maximums as indicated by the solid circles designated plus and minus in FIG. 4. The other orthogonal mode will assume a position such that it has a null at the coupling means 23. Thus, the mode of resonance having the null at the coupling means 23 will not be substantially coupled to the load and therefore will not be heavily loaded and it will have a high Q resonance, whereas the mode which is heavily coupled to the load will have a relatively low Q resonance.

As the tube is tuned in frequency, the power output spectrum will be as shown in FIG. 5. More particularly, it will be found that as the tube is electronically tuned through its power spectrum the high Q mode will come into resonance in the center of the band and will couple power from the desired mode into its mode which is uncoupled from the load and therefore the power output spectrum will have a dip 28 in the center of its power output spectrum.

Due to the orthogonal nature of the uncoupled mode, it will have maximum intensities at the null points 27 of the coupled mode. Thus, this undesired mode may be substantially perturbed without interfering with the desired or coupled mode by providing means for perturbing this undesired mode at its point of maximum intensity. Accordingly, in the present invention a short circuit is provided on the periodic circuit 12 at a point which corresponds to a null of the coupled mode and a maximum of the uncoupled mode. The short circuit is indicated at 29 on FIG. 4. This short circuit occurs at a point in space from the output coupling loop 23 taken about the cavity 22. This short circuit at 29 very substantially perturbs the undesired mode, shifting its frequency out of the operating band of the tube, as indicated by the arrow of FIG. 5.

The undesired mode of course has two nulls in space quadrature from the output coupling loop 23 at both positions 27; however, if a second short circuit is provided at the second point 27, this serves to then divide the circuit into two resonant sections which is generally undesirable as it interferes with the transfer of energy around the circuit. Although, in a preferred embodiment, the perturbing means 29 conveniently takes the form of a short circuit on the interdigital line, it is contemplated that other heavily perturbing means may be used for suppressing or perturbing the undesired mode without substantially interfering with the desired mode. More specifically, it is contemplated that a lossy material could be placed on the circuit at this point or a resonant lossy member could be employed in the cavity 22 for absorbing the power of the undesired mode and suppressing it from oscillation. However, it is preferred that a short he provided on the circuit as this very effectively perturbs the undesired mode. A specially convenient way to obtain a short circuit is shown in FIG. 6. In this instance a conductive element 31, as of copper, merely interconnects the axial end of one of the fingers to the opposed wall of the cavity 22, thereby forming a short across the circuit.

Referring now to FIG. 7, there is shown an (0-5 diagram depicting a first 0 mode of operation and some of the adjoining half and integral modes for an X band tube constructed according to the features of the present invention. In this particular instance, the low frequency cutoff of the circuit determined by the dimensions of cavity 22 was selected at 4.3 gc. and corresponded to the 1r mode. The upper cutoff frequency of the periodic circuit 21 was selected by dimensioning the lengths of the fingers of the interidigtal line portion to have an upper cutoff frequency of approximately 70 gc. This occurred when the finger length was approximately 0.084". By separating the high and low frequency cutofi points by such a wide frequency separation, a very steep dispersion characteristic 6 was obtained near the 1r mode. This steep characteristic permitted substantial frequency separation of the modes and, in particular, the first 0 mode was found to have a frequency of approximately 9.2 gc. As previously mentioned in regard to FIGS. 1 and 2, the short circuit element at 29 introduced one-half modes at positions on the dispersion curve corresponding to halfway, in 5, between the integral modes. These half modes are shown on the diagram of FIG. 7. The first (1/2) mode had a resonant frequency of 7.8 gc. and the next highest (3/2) mode occurred at a resonant frequency of over gc. With the anode circuit 12 heavily loaded and exhibiting a Q of approximately 15 to 20, an electronically tunable bandwidth of 1000 mc. between synchronous voltages of V and V was obtained. The circuit had 24 periodic elements and yielded a power output of approximately 2.5 watts over the tunable band.

Since many changes could be made in the above construction and many apparently widely dilTerent embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a voltage tunable magnetron means forming a cold cathode sole electrode, means forming an anode electrode surrounding said cathode sole electrode and defining an annular magnetron interaction region between said anode and cathode, means forming a reentrant periodic magnetron interaction circuit formed in said anode adjacent said interaction region, thermionic electron emitter means axially spaced from said sole electrode for axially injecting a stream of electrons into said interaction region for cumulative electronic interaction with wave energy on said periodic circuit to produce an output signal, means for coupling the output signal from a desired mode of oscillation on said circuit to a suitable load and for heavily loading said interaction circuit to provide a loaded interaction circuit Q of less than 50, said periodic circuit being capable, as coupled to the load, of supporting an interferring mode of oscillation at the signal frequency, and

means on said anode circuit at a position of relatively low amplitude of said desired mode and relatively high amplitude of said undesired mode for perturbing said undesired mode and removing it from substantial interference with the desired mode, said perturbing means being located at only a single location as taken around the circumference of said anode circuit.

2. The apparatus according to claim 1 wherein said perturbing means comprises a low impedance member shunting across said interaction circuit for perturbing the frequency of said undesired mode away from the center of the operating band of the tube.

3. The apparatus according to claim 2 wherein said low impedance member comprises a short circuit.

4. The apparatus according to claim 1 wherein said periodic interaction circuit is an interdigital line, and said perturbing means a short circuit on said line.

5. The apparatus according to claim 1 wherein said anode circuit includes a cavity resonator portion surrounding said periodic circuit, and said perturbing means is locate-d on said circuit in approximate quadrature-spaced relation from said output coupling means as taken around the circumference of said re-entrant anode circuit.

6. The apparatus according to claim 5 wherein said cavity resonator is dimensioned for operation on the TM mode at the output signal frequency.

7. The apparatus according to claim 6 wherein said periodic circuit portion is an interdigital line having interdigitated finger portions.

8. The apparatus according to claim 7 wherein said perturbing means is a short circuit element on said interdigital line interconnecting adjacent interdigitated fingers thereof.

References Cited UNITED STATES PATENTS 2,565,387 4/1951 McCarthy 315-3973 X 2,635,211 4/1953 Crawford et al. 315-39.73 2,997,624 8/1961 Peters 315-3963 X HERMAN KARL SAALBACH, Primary Examiner. SAXFIELD CHATMON, JR., Assistant Examiner.

US. Cl. X.R. 

50. THE COMPOSITE INTERACTION CIRCUIT. AS COUPLED TO THE LOAD, IS CAPABLE FOR SUPPORTING AN INTERFERING MODE OF OSCILLATION AT THE SIGNAL FREQUENCY. A WAVE ENERGY PERTURBING MEANS, SUCH AS A CONDUCTIVE ELEMENT OR LOSSY ELEMENT, IS PROVIDED ON THE LOADED INTERACTION CIRCUIT AT A POSITION OF RELATIVELY LOW AMPLITUDE OF THE DESIRED MODE AND RELATIVELY HIGH AMPLITUDE OF THE UNDESIRED MODE FOR PERTURBING THE UNDESIRED MODE AND REMOVING IT FROM SUBSTANTIAL INTERFERENCE WITH THE DESIRED MODE. SUCH PERTURBING MEANS IS LOCATED AT ONLY A SINGLE LOCATION AS TAKEN AROUND THE CIRCUMFERENCE OF THE ANODE CIRCUIT TO PREVENT DIVIDING THE CIRCUIT INTO TOW OR MORE RESONANT SECTIONS WHICH COULD INTERFERE WITH TRANSFER OF ENERGY AROUND THE CIRCUIT. 